1. Field of the Invention
This invention relates to a hydrophilic porous membrane, to a method for the production of the membrane, and to a plasma separator using the hydrophilic porous membrane. More particularly, it relates to a hydrophilic porous membrane which exhibits prominent dimensional stability and strength while in use and succumbs sparingly to degradation of membrane qualities, to a method for the production of the membrane, and to a plasma separator using the hydrophilic porous membrane.
2. Description of the Prior Art
The separation of substances as by filtration or dialysis using macromolecular porous membranes has found utility in numerous applications owing to its advantages in operability and economy. Generally, in the separation of aqueous media such as aqueous solutions and blood, either hydrophilic porous membranes are used or hydrophobic porous membranes which have undergone a treatment for impartation of hydrophilicity are used.
As hydrophilic porous membranes, porous membranes of cellulose derivatives, particularly cellulose acetate, which possess high water permeability are in wide prevalence. These cellulose derivatives, however, are deficient in resistance to acids, alkalis, and organic solvents and they also have the drawback of readily deforming on exposure to heat and pressure. Thus, they are used only under heavily limited conditions. Further, the cellulose derivatives, on contact with water, swell. When a porous membrane made of such a cellulose derivative as described above incorporated a device and put to use, this phenomenon of swelling causes the porous membrane to deform itself and furrow its surface possibly to the extent of obstructing flow paths formed in the device, inducing the phenomenon of channeling, and preventing the membrane from discharging its function sufficiently. When a porous membrane made of cellulose acetate is adopted in the biomedical field as for plasma separation, this membrane is suffered to activate the complementary elements in the body fluid and entail difficulties even from the standpoint of bioadaptability.
In contrast, hydrophobic porous membranes generally enjoy high strength, excellent chemical resistance and other highly desirable properties inherent in hydrophobic macromolecules which form the membranes and they do not swell in water and, therefore, entail none of the difficulties mentioned above. Under widely practiced conditions of filtration (for example, a filtration pressure of not more than 1 kg/cm.sup.2), however, the porous membranes do not permit passage of water through their continued pores. Thus, these membranes must be treated so that their pores will acquire a hydrophilic surface. As means of treatment for producing the hydrophilic surface, the organic solvent-water substitution method which produces this surface by immersing the hydrophobic porous membrane in an organic solvent such as, for example, ethanol or other alcohol and then displacing the organic alcohol contained in the membrane with water and the coating method which produces the surface by coating the hydrophobic membrane with a surfactant or with glycerol or polyvinyl alcohol or with a hydrophilic polymer have been well known. Unfortunately, these methods are incapable of imparting permanent hydrophilicity to the membrane. Each time the membrane has lost this hydrophilicity, therefore, it must be treated for restoration of the hydrophilicity. This inevitable repetition of the treatment adds to the complexity of the use of this membrane. To be specific, the membrane treated by the former method loses the hydrophilicity after it is dried and the membrane treated by the latter method loses the hydrophilicity after the coating compound has flowed out of the membrane. Many attempts are being made to impart semipermanent hydrophilicity to hydrophobic porous membranes. They encounter many problems, however, because they are porous by nature. No method has yet been established which is capable of permitting perfect impartation of the semipermanent hydrophilicity. The method which imparts a hydrophilic group to the surface of a membrane by a treatment using an aqueous solution of an Laid-Open such as sodium hydroxide or potassium hydroxide (Japanese Patent Laid-Open No. SHO 58(1983)-93,734) has the possibility of the membrane losing strength by the action of the alkali and entails the problem of difficult control of the conditions of the treatment. The method which resorts to the steps of immersing a hydrophobic porous membrane in an alcohol, then treating the impregnated membrane with an aqueous water-soluble polymer solution, drying the treated membrane, and thereafter treating the membrane as with heat or a radiant ray thereby insolubilizing the water-soluble polymer remaining on the membrane (Japanese Patent Publication No. SHO 54(1979)-17,978, Japanese Patent Laid-Open No. SHO 56(1981)-38,333, etc.) has a strong possibility of consuming a great deal of time during the step of immersion in the alcohol through the step of substitution with the aqueous polymer solution, degrading the membrane strength under the influences of heat or radiant ray during the treatment for insolubilization, and causing variation of diameters of pores in the membrane. Moreover, this method has room for solution with respect to equipment, safety, and cost.
The plasma modification method accomplishes the impartation of hydrophilicity by irradiating the surface of a hydrophobic macromolecular membrane with a plasma. Since the hydrophilicity created by this treatment is gradually degraded with elapse of time (Professor Ikada, Kyoto University "Kogyo Zairyo: Vol. 31, No. 7, pages 62-69), this method has a critical disadvantage that it is incapable of imparting lasting hydrophilicity.
Further, the practice, of imparting hydrophilicity by producing points of activity on the surface of a hydrophobic macromolecule and graft polymerizing a hydrophilic monomer on the macromolecule has found acceptance. When the graft polymerization is effected by using such a radiant ray as electron ray or .gamma. ray which possesses a high penetrating force, this method suffers from loss of the strength of basic material and entails unsolved problems concerning equipment, safety, and cost. Further, the graft polymerization causes the organic macromolecule which has nothing to do with the graft polymerization to form an ungrafted homopolymer. Thus, this method has much to be desired regarding effectiveness and efficiency. Studies are now devoted to development of a graft polymerization method using a plasma, chiefly for the modification of surface of macromolecular basic materials. By the method using ordinary plasma, since the penetrating force of plasma is weak as compared with that of the radiant ray, the surface of such minute openings as pores inside a porous membrane is not easily treated effectively. It has been held that the depth from the outer surface of a macromolecular material in which the treatment with plasma produces the desired effect thereof is only several microns at most (pages 11-13 of Japanese Patent Laid-Open No. SHO 56(1981)-38,333 and Kogyo Zairyo, Vol. 31, No. 7, pages 62-69). Conceivably, impartation of hydrophilicity deeply into the interior of a porous material may be obtained by amply increasing the amount of grafting. When the amount of grafting is increased, however, there ensues the disadvantage that the pores in the porous membrane are clogged so heavily as to prevent the membrane from providing ample permeating of water or the pores have their diameters so heavily varied as to impede the membrane from fulfilling the function of a molecular sieve (separation membrane). Further, since the membrane is suffered to swell so much under a moist condition, the membrane incorporated in a device and put to use entails a critical disadvantage that the membrane will induce the phenomenon of channeling as described above. Recently, the case of application of plasma to the treatment for impartation of hydrophilicity to a hydrophobic membrane has been reported (Japanese Patent Laid-Open No. SHO 59(1984)-160,504). Unlike the conventional plasma treatment which aims to cause reaction of a gaseous monomer in the presence of plasma, this method makes use of the so-called plasma graft polymerization which comprises irradiating a given basic material with plasma thereby forming points of activity such as radicals on the surface of the basic material and then subjecting the basic material to polymerization in an aqueous monomer solution in the absence of plasma. This method is claimed to permit desired control of the graft polymerization ratio. This method, however, has not been demonstrated to be capable of permitting impartation of perfect hydrophilicity to the surface of pores in the porous membrane in a relatively low graft ratio enough to preclude the drawbacks mentioned above. When the graft polymerization is effected with any of the hydrophilic unsaturated monomers enumerated in the specification, the grafted layer is formed toward the outside on the porous membrane as the basic material because the unsaturated monomer has poor affinity for the hydrophobic macromolecule forming the basic material. It is considered that since the monomer is supplied abundantly in the form of a solution, the growth of graft chains proceeds vigorously and the development of the grafted layer near the outer surface of the porous membrane with relatively rich distribution of points of activity produced by plasma irradiation occurs very quickly as compared with the develpment of the grafted layer deep inside the membrane. It naturally follows that the graft layer fully developed near the outer surface of the porous membrane before the impartation of the perfect hydrophilicity proceeds thoroughly to the surface of pores inside the porous membrane will clog the pores in the porous membrane. Even from the commercial point of view, this method has the disadvantage that the monomer is heavily wasted and the operability is lowered because the reaction occurs in the liquid phase. It has been demonstrated by the inventors that when the same mechanism of polymerization initiated by plasma is applied to the combination of high mutual affinity between the substrate of N,N-dimethyl acrylamide and the porous membrane of polypropylene, the grafted layer attains its development deeply inside the substrate. There is the possibility, however, that since the monomer is supplied in the form of a solution to the reaction the grafted layer will develop so much as to lower the strength of the substrate. This method can not be called very suitable for the purpose of imparting hydrophilicity to a hydrophobic porous membrane intended for use in filtration or dialysis.
This invention aims to provide a novel hydrophilic porous membrane, a method for the production thereof, and a plasma separation using the hydrophilic porous membrane. This invention also aims to provide a hydrophilic porous membrane exhibiting prominent dimensional stability and strength while in use and succumbing only sparingly to degradation of membrane qualities, a method for the production thereof, and a plasma separator using the hydrophilic porous membrane. This invention further aims to provide a hydrophilic porous membrane possessing out-standing water permeability owing to the impartation thereto of lasting hydrophilicity at no sacrifice of highly desirable physical properties inherent in a hydrophobic porous membrane, method for the production thereof, and a plasma separator using the hydrophilic porous membrane. Further, this invention aims to provide a hydrophilic porous membrane enjoying great advantages in terms of equipment, safety and cost, a method for the production thereof, and a plasma separator using the hydrophilic porous membrane.